Download Pyrodiversity vs Biodiversity

Pyrodiversity
vs Biodiversity
DALE NIMMO
New research challenges conventional wisdom that the creation of a diverse mosaic of fire histories
benefits biodiversity.
ire is a part of life in Australia. It can be an agent of
destruction and loss, and it can promote regeneration
and new life.
While most terrestrial ecosystems in Australia are fire-prone,
our understanding of the relationships between fire regimes
and the Australian flora and fauna remains limited. Such understanding is critical because inappropriate fire regimes – such
as burning too little or too often – have been implicated in the
decline and extinction of a range of native species.
A large team of scientists and volunteers have set out to
rectify this knowledge gap by undertaking a major study across
the Mallee region where Victoria, South Australia and New
South Wales meet. What we found challenges prevailing
wisdom, and suggests that fire management regimes will need
to vary between ecosystems to maintain the widest diversity of
species.
F
The Ecological Role of Fire
Picture a site that has recently burned: the canopy scorched
and its dead leaves starting to fall; bare ground where the leaf
litter has been consumed by fire; and logs blackened to coal on
the ground. What looks like a harsh, uninhabitable landscape
can in fact be the preferred habitat for species that like open,
structurally simple areas and do not rely on thick vegetation
for shelter – burrowing species, for instance. These species will
be most common in the recently burned landscape.
Now imagine weeks, months and years passing. Grass and
shrubs gradually return to cover the ground layer. The race for
sunlight is on as trees germinate from seeds, resprout from
lignotubers or simply regrow their branches and leaves from
individuals that survived the fire. The pace of this change will
vary substantially between ecosystems.
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These changes to vegetation bring about further changes to
the penetration of sunlight through the returning canopy and
the availability of habitat resources such as tree hollows, logs and
leaf litter. Now, animal species that prefer open habitats are
finding it tough. Instead, species that shelter under logs or in
leaf litter are becoming more abundant, potentially outcompeting other species for food and other resources.
As even more time passes, trees may senesce and a new raft
of changes will favour yet another group of plant and animal
species.
Patch-Mosaic Burning
Because ecosystems change over time following fire (although
at vastly different rates depending on the ecosystem), an important consideration in fire ecology is how to accommodate all the
needs of these different species.
A widely advocated approach is “patch-mosaic burning”.
The emphasis of patch-mosaic burning is to maintain a range
of different fire histories across a landscape – for instance, some
patches that have recently burned, some not burned for a decade,
and some patches of much greater age since the last fire. The aim
is that by creating a mosaic, the requirements of all species
might be met, allowing species to co-exist despite their differing
habitat preferences.
A corollary of this logic is that a diverse mosaic of fire ages
will accommodate the needs of more species, and therefore the
diversity of fire ages within a landscape can be considered a
surrogate for the amount of biodiversity. This logic has been
captured in the phrase “pyrodiversity begets biodiversity”.
Patch-mosaic burning has become common in fire management around the world, but particularly in South Africa and
Australia, where government agencies often work towards
Fire is a key part of ecosystems in the Mallee.
creating a fine-scale patch-mosaic of fire ages. However, despite
its widespread application, until recently very little empirical
research had been done to test the assumption that pyrodiversity does indeed promote biodiversity.
Fire in the Mallee
The Mallee Fire and Biodiversity Team sought to fill this knowledge gap by undertaking a large-scale research project in the
Murray–Mallee region of south-eastern Australia.
Mallee vegetation extends across vast areas of semi-arid
southern Australia, and is characterised by multi-stemmed
“mallee” eucalypt trees and red, sandy soils.
As you move through mallee landscapes, the dramatic effects
of fire become clear. Recently burned areas can be almost devoid
of vegetation, apart from the dead stems of mallee eucalypt
trees that remain as legacies of the pre-fire ecosystem. Older
areas have a sparse canopy of eucalypts and often an understory
of shrubs, chenopod plants and spinifex grass. These landscapes
hold a rich fire history as mallee vegetation is highly fire-prone,
with frequent, very large wildfires (>100,000 ha) occurring
every 10–20 years within the Murray–Mallee region.
At around 5 metres in height, mallee trees are relatively short
for eucalypts. As such, the distance between the ground fuel –
such as leaf litter, bark and spinifex clumps – and the tree
canopy is relatively small. Consequently, fires in mallee vegetation are usually “crown” fires that consume most aboveground vegetation, and are therefore stand-replacing. The
Credit: Peter Teasdale
mallee trees regrow after fire from their root system, which is
buffered from the fire and remains alive underground. Other
plant species regrow by resprouting or from seeds.
Although this area is vast and may seem pristine at first
glance, many changes have taken place over the past 200 years
that have created novel ecological conditions.
First, large areas have been cleared for grazing and cropping,
so formerly continuous tracts of vegetation are now fragmented
and dispersed.
Second, many species that once inhabited the Murray–Mallee
are no longer there. These include many small- and mediumsized mammals, such as brush-tailed bettongs, pig-footed bandicoots and the lesser stick-nest rat. These species declined quickly
as native predators such as quolls and dingoes were replaced
by feral cats and red foxes.
Despite these changes, the Murray–Mallee region still supports
a large number of rare and fascinating animal species. Native
mammals include the mallee ningaui, Mitchell’s hopping mouse,
and both the little pygmy possum and western pygmy possum.
Western grey kangaroos are the most common large mammal.
The extremely diverse reptile fauna includes approximately
60 species, including a range of dragon lizards, geckoes, skinks,
legless lizards and snakes. The bird fauna is also impressive,
including iconic and threatened species such as Major Mitchell’s
cockatoo, malleefowl and the mallee emu-wren.
Fire in the Murray–Mallee region is actively managed for
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vation, and sometimes in accordance with the principles of
patch-mosaic burning.
All of these factors mean that these landscapes are ideal for
researching the ecological role of fire.
The Mallee Fire and Biodiversity Project
The Mallee Fire and Biodiversity Project was undertaken by a
team of scientists and PhD students from La Trobe and Deakin
universities in collaboration with government agencies, private
landowners and several conservation organisations from New
South Wales, Victoria and South Australia. In addition, more
than 100 volunteers assisted with field work for this project,
including installing and checking pitfall traps.
The study area encompassed 104,000 km2 , an area approximately the size of Tasmania. Because of the large number of
researchers involved we were able to study a range of taxonomic
groups, including reptiles, mammals, birds, selected invertebrates and plants.
The team established pitfall traps at 280 sites for sampling
reptiles, small mammals, scorpions and centipedes. Birds and
termites were monitored at 560 sites, and vegetation surveys were
conducted at more than 800 sites. Overall, it was one of the
largest surveys of its kind.
This survey effort resulted in about 10,000 reptile records,
more than 20,000 bird records, about 1500 records of small
mammals and nearly 500 centipedes. These large datasets put
us in a great position to enhance our understanding of the fire
ecology of the mallee.
Fire over the Long Term
Before we could test the hypothesis that pyrodiversity begets
biodiversity, it was important to determine whether fire history
affects biodiversity in our study region. If fire history does not
affect the distribution of plants and animals in the region then
fire management, including patch-mosaic burning, will have
little effect on the region’s biodiversity.
One obstacle we faced was that, as with most regions, systematic knowledge of the fire history of the Murray–Mallee went
back only to the 1970s, when satellite imagery became available and was used to document landscapes in a consistent and
regular way. The “footprint” of fires is often distinct in satellite images, allowing fire ecologists to map fires through time.
However, this reliance on satellite imagery left hidden the age
of fires that occurred long before the 1970s.
Our team developed statistical models of the growth rates
of eucalypt stems following fire to determine the age of sites
up to 110 years after fire. We found that the effects of fire on
ecosystems last far longer than we had appreciated. Changes
in resources that are critical to fauna, such as the cover of important plant species and the abundance of tree hollows, continued
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Fauna of the Murray–Mallee region (clockwise from above):
a western pygmy possum, a beaded gecko, a southern legless
lizard, yellow-plumed honeyeaters and a mallee ningaui with young.
to occur throughout the entire 110-year period.
We found that many of the animal species that are affected
by fire are tightly linked to these resources, so they are unlikely
to occur where their favoured resources are scarce. Consequently, the effects of fire can continue to influence some species
for a century following fire. For instance, the Murray striped
skink and the yellow-plumed honeyeater are both most common
at sites that have not burned for 60–100 years.
Species that depend on spinifex hummocks, such as the
mallee emu-wren, mallee ningaui and southern legless lizard,
reach their greatest abundance 20–50 years post-fire when large
amounts of spinifex cover occur.
Now that we had established that fire is an important and
long-term driver of plants and animals in the region, the next
question was whether a more diverse patch-mosaic does indeed
have more biodiversity.
Comparing Whole Fire Mosaics
When studying ecological processes, it is important that the
spatial scale of a study matches the scale of the process to be
investigated. In this instance we were interested in whether
landscapes that had a diverse fire history (i.e. many fire ageclasses present) had more biodiversity than those with more
uniform fire histories. Thus we needed to compare biodiversity
between entire patch-mosaics that differed in their fire history.
This meant sampling both fire history and biodiversity for
entire landscapes.
Credit: Lauren Brown
Credit: Simon Watson
Credit: Lauren Brown
Credit: Lauren Brown
Credit: Mallee Fire and Biodiversity Team
Our study design included 28 landscapes, each 4 km in diameter, that represented a gradient in the diversity of their fire
history. Some landscapes had a single fire-age, such as being
completely burned at a single time recently; other landscapes
had two fire age-classes; and yet other landscapes had up to five
or six fire ages present, representing a fine-scale fire mosaic.
This sampling approach, known as “whole-of-landscape”
sampling, allowed us to directly compare the biodiversity in
these 28 landscapes to examine whether those with more fireages (i.e. those that are more “pyrodiverse”) did in fact have
more species.
Does Pyrodiversity Beget Biodiversity?
We found little evidence that landscapes with more diverse
fire histories had greater biodiversity for any of the taxonomic
groups studied: that is, landscapes that had a more diverse fire
history did not have more species of invertebrates, reptiles,
small mammals or birds.
Instead, an important driver for all taxonomic groups was the
spatial extent of ecologically important fire age-classes. For
example, the species richness of all birds, as well as the richness
of threatened bird species such as the malleefowl and mallee
emu-wren, was positively associated with the amount of longunburned vegetation in the landscape (which was classified as
mallee vegetation unburned for more than 34 years).
Likewise, the species richness of small mammals and reptiles
was not related to the diversity of fire age-classes. Individual
species of mammals and reptiles were often more common in
landscapes with a greater area of a preferred fire-age. For instance,
the western pygmy possum was more common in landscapes
with a greater area of long-unburned vegetation, while the
mallee dragon, southern legless lizard and the mallee ningaui –
all of which use spinifex hummocks for shelter – were less
common in landscapes with a large proportion of recently
burned vegetation, where Spinifex is less common.
Our findings challenge the dominant paradigm that the
creation of diverse fire mosaics will enhance biodiversity, but
what do we propose to replace it?
We suggest that the focus for managing fire mosaics should
shift from simply creating a diversity of fire ages in landscapes
to a strategic focus that seeks to ensure an optimum mix of ageclasses at the regional scale. Researchers from the team are
currently working on tools that can help determine this optimal
mix.
Importantly, the optimal mix of fire-ages will differ substantially between ecosystems due to differences in the amount of
time over which the recovery of habitat resources occurs
following fire, the different species that occur within ecosystems, and their strategies for dealing with fire. As such, it is
critical that targets for the optimal mix of age-classes be set on
a ecosystem-by-ecosystem basis, and be informed by knowledge of how fire affects the distribution of resident species.
Dale Nimmo is an Alfred Deakin Postdoctoral Research Fellow at Deakin University’s School
of Life and Environmental Science.
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